1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image formed on an image carrying member to convert it into a visible image. More particularly, the present invention relates to a developing device which uses a magnetically attractable developer for developing an electrostatic latent image whereby a thin film of uniformly charged developer is formed on a developer carrier prior to the application of the developer to the latent image at a developing station. More specifically, the present invention relates to a developing device using the so-called single component toner, which is particularly suited for use in an imaging machine such as an electrophotographic copying machine.
2. Description of the Prior Art
In various types of imaging machines, such as electrophotographic copiers, electrostatic recording machines and laser printers, a developing device is normally provided to develop a latent image formed on an image carrier, such as a photosensitive member, thereby converting the latent image into a visible image. In general, there are two categories of developing devices for use in such imaging machines: one category uses magnetically attractable toner, or often referred to as "single-component developer", and the other category uses the so-called two-component developer comprised of a mixture of toner particles and carrier beads. In either category of developing devices, usually the formation of a thin film of developer is desired or required before application to a latent image for its visualization. Particularly, in the case of a developing device utilizing a single-component developer comprised of magnetically attractable, electrically resistive toner particles, the developer is required to be forcibly changed to a predetermined polarity and level uniformly, and it is important that a thin film of developer may be formed as a preparation for application to a latent image to be developed.
In forming a thin film of uniformly charged toner particles on the surface of a developer carrier, which is driven to advance through a developing station where a latent image on an image carrier is developed with the application of the toner particles, it has been proposed to employ a pressure plate comprised of a resilient blade or a magnetic material and disposed with its free end portion in pressure contact with the surface of the developer carrier so as to form a thin toner film having a desired thickness and uniform charge due to frictional charging at the contact between the developer carrier and the pressure plate. Since the thickness of resulting thin toner film depends upon the contact condition between the developer carrier and the pressure plate, the surface condition of the pressure plate in contact with the developer carrier is vitally important, especially for securing a non-variant film forming performance for a long period of time.
FIG. 1a schematically illustrates a typical prior art developing device which utilizes magnetically attractable, electrically insulating toner particles as a developer, and it comprises a developing sleeve 2 which is driven to rotate in the direction indicated by the arrow and which transports toner particles 1 as carried on its peripheral surface as magnetically attracted thereto by means of a multi-pole magnet disposed inside of the sleeve and driven to rotate in the direction indicated by the arrow. Also provided is a pressure plate 3 whose free end portion is pressed against the peripheral surface of the developing sleeve 2. Thus, as the sleeve 2 is driven to rotate in the direction indicated by the arrow, the toner particles 1 attracted to the peripheral surface of sleeve 2 become compressed between the sleeve 2 and the pressure plate 3 at a point P thereby allowing to form a thin film of uniformly charged toner particles. However, after this arrangement is used for a relatively long period of time, the toner particles 1 tend to accumulate at an entrance section P.sub.1 and an exit section P.sub.2 fore and aft of the contact line P between the sleeve 2 and the plate 3. Under this condition, the accumulated toner particles tend to stick to the surface of the pressure plate 3 as shown in FIG. 1b. In the case of the developing device shown in FIG. 1a, since the tip end of plate 3 extends further downward from the contact line P between the sleeve 2 and the plate 3, there is formed a pair of stuck toner ridges 1a and 1b on the plate 3 fore and aft of the contact line P, as shown in FIG. 1b. It is thus quite important from a practical viewpoint to devise a structure which would help prevent the formation of such stuck toner ridges and keep the surface of the pressure plate as smooth as possible at all times.